The radio communication system according to IEEE 802.16 standard is known as one of radio communication techniques to allocate radio resources. The IEEE 802.16 defines a technique of establishing a wireless MAN (Metropolitan Area Network) as a wide area network to mutually connect LANs (Local Area Networks) in a metropolitan area or a specific area by wirelessly connecting a communication carrier to user's homes without using telephone lines or optical fiber lines. According to this technique, a single radio base station may cover an area of about 50 km in radius at a transmission rate of about 70 megabits per second at the maximum.
Under current circumstances, standardization of 802.16d specifications (802.16-2004) mainly for fixed communication application and 802.16e specifications (802.16e-2005) for mobile communication application is progressing in the IEEE 802.16 Working Group.
FIG. 14 illustrates an example of a configuration of a conventional radio communication system. The radio communication system 1 in this figure includes a radio base station (BS) 2 and at least one mobile station (MS) 3. Radio signals are mutually transmitted/received therebetween, so that communication in downlink DL (from BS to MS) and uplink UL (from MS to BS) is performed. In the IEEE 802.16e, transmission/reception of the radio signals is performed by using radio frames generated by using OFDMA (Orthogonal Frequency Division Multiplexing Access).
FIG. 15 illustrates an example of a radio frame compatible with the IEEE 802.16e. In the figure, the horizontal axis indicates a time direction expressed in units of symbols, whereas the vertical axis indicates a frequency direction expressed by logical subchannels which are units in which a plurality of subcarriers are grouped. FIG. 15 illustrates an example where TDD (Time Division Duplexing) is used as a method for multiplexing uplink (UL) and downlink (DL). The radio frame 4 includes a DL subframe in the first half and a UL subframe in the second half. The DL subframe includes a preamble (symbol k); FCH (Frame Control Header (symbol k+1): including information such as the length of DL-MAP, encoding method, and the number of repetitions); DL-MAP (indicating an allocation state of bursts (DL bursts #1 to #6 in FIG. 15) on the DL subframe); UL-MAP (showing an allocation state of bursts (Ranging subchannel Rs (symbol k+17 to k+26) and UL bursts #1 to #5 in FIG. 15) on the UL subframe); and DL bursts (store data and message addressed to the MS 3). Here, in the UL subframe, the MS 3 transmits a control message or UL data to the BS 2 by using a certain area such as the Ranging subchannel Rs or an UL burst based on the mapping illustrated in the UL-MAP (see broken-line right arrow in FIG. 15).
As is clear from FIG. 15, in the IEEE 802.16e, every UL communication from the MS 3 is performed based on the UL-MAP generated by the BS 2. Thus, in the IEEE 802.16e, several scheduling types are defined as a communication control method for allowing the MS 3 to receive allocation of a UL burst for UL data transmission from the BS 2.
FIGS. 16A and 16B illustrate states of occurrence of UL data in the MS 3 and transmission control between the BS 2 and the MS 3. Various applications of Web, audio, video, etc. are executed in the MS 3. Here, assume a variable-data-rate-type application having high real-time performance of data and having temporal variations in the amount of data occurred, as in a voice or TV conference in which “silence compression” is performed.
In a period when UL data occurs, the BS 2 needs to allocate a UL burst for transmitting the UL data to the MS 3. On the other hand, in a period when no UL data occurs, there is no need to allocate a UL burst. In an ordinary case, a period when data occurs is independent in every application of the MS 3. Thus, in a period when no UL data occurs in an MS, radio resources are allocated to another MS in which UL data occurs, so that the usage efficiency of the radio resources may be enhanced and a UL throughput in the entire BS 2 may be enhanced.
As described above, in order to efficiently use the radio resources, it is necessary to dynamically and quickly recognize the necessity of allocation of a UL burst and reflect the recognition on the scheduling in the BS 2 in accordance with presence/absence of UL data in the MS 3. As a communication control method suitable for UL connection having such a real-time variable-rate-type traffic characteristic, a scheduling type called “rtPS (real-time polling service)” is defined in the IEEE 802.16e.
FIG. 16B illustrates a sequence of UL transmission control using the above-described rtPS between the BS 2 and the MS 3. In the rtPS, polling is regularly performed from the BS 2 to the MS 3. Here, polling in the IEEE 802.16e means allocation by the MS 3 to the BS 2 of a UL burst having a size (6 bytes) necessary to transmit a BR (Bandwidth Request) message to make a UL burst allocation request. Each of the UL burst allocated as polling and the UL burst allocated to transmit UL data of an application is constituted by a slot area on the UL subframe specified by the UL-MAP as illustrated in FIG. 15, although the sizes of those bursts are different from each other.
When polling is performed by the BS 2, if UL data to be transmitted occurs in the MS 3, the MS 3 transmits a BR message to the BS 2 by using the UL burst allocated through the polling so as to notify the BS 2 of the size (in units of bytes) of the UL data to be transmitted from the MS 3.
The BS 2 that has received the BR message selects MSs to which UL bursts are to be allocated and determines the sizes of the bursts to be allocated based on scheduling by a scheduler in view of also BR messages from other MSs.
As a result of the scheduling, the BS 2 generates the above-described UL-MAP and broadcasts the UL-MAP to all the MSs in the DL subframe of the subsequent radio frame 4. Each MS analyzes the UL-MAP received through the broadcast. If a UL burst is allocated to itself (MS), the MS transmits UL data by using the UL burst. This UL burst has a size larger than that of the UL burst used for polling in an ordinary case.
In this way, transmission of UL data in the rtPS is controlled between the BS 2 and the MS 3 in the following four steps S1 to S4.
S1: From BS to MS: Polling (allocation of UL burst for BR message) is executed.
S2: From MS to BS: BR message is transmitted.
S3: From BS to MS: UL burst for data transmission is allocated.
S4: From MS to BS: UL data is transmitted.
In the example illustrated in FIG. 16B, the above-described transmission control including steps S1 to S4 is performed in each of the processes where UL traffic occurs in the MS 3 (P2 to P7, P10, and P11 on the upper side in FIG. 16B) On the other hand, in the processes where no UL traffic occurs (P1, P8, and P9 on the upper side in FIG. 16B), polling in step S1 is performed but the MS 3 does not transmit a BR message to the BS 2 in step S2 because there is no UL data to be transmitted.
In the IEEE 802.16e, negotiations are performed between the BS 2 and the MS 3 about the scheduling type to be used and traffic parameters for scheduling before UL connection is established at the start of communication. For example, in the rtPS, negotiations are performed about the following respective traffic parameters.
Maximum Latency (maximum allowable delay)
Minimum Reserved Traffic Rate (minimum guaranteed rate)
Maximum Sustained Traffic Rate (maximum allowable rate)
Traffic Priority (traffic priority)
Request/Transmission Policy (attribute/policy about BR and transmitting process)
Unsolicited Polling Interval (polling interval)
The BS 2 performs scheduling an allocation of a UL burst for connection of the MS 3. The interval of polling is defined by “Unsolicited Polling Interval”. “Minimum Reserved Traffic Rate” and “Maximum Sustained Traffic Rate” are parameters about a transmission rate of UL data. The BS 2 controls allocation of UL bursts so that the traffic rate does not surpass “Maximum Sustained Traffic Rate”, which is the maximum allowable rate, while guaranteeing “Minimum Reserved Traffic Rate” on average.
According to the above-described UL communication control method, UL radio resources may be efficiently used by dynamically controlling allocation of a UL burst in accordance with presence/absence of UL data in the MS 3.
However, the inventor has found that the following inefficiency occurs depending on an occurrence state of UL data in the MS 3.
As illustrated in FIGS. 16A and 16B, for example, UL data intermittently occurs in variable-data-rate-type traffic using the rtPS. When UL data occurs, the UL data is continuously transmitted temporarily at the maximum allowable rate defined by the traffic parameter. According to the example illustrated in FIGS. 16A and 16B, UL data is transmitted at the above-described maximum allowable rate in P3 to P7 on the upper side in FIG. 16B.
At this time, polling from the BS 2 to the MS 3 is constantly performed in each of P1 to P11 in FIG. 16B based on the polling interval parameter regardless of presence/absence of UL data. Therefore, the BS 2 constantly performs polling also in processes P3 to P7 on the upper side in FIG. 16B where UL data is transmitted at the maximum allowable rate. In response to this, the MS 3 transmits a BR message at each time so as to request allocation of a UL burst.
However, in the state where transmission of UL data is continued as in processes P3 to P7 on the upper side in FIG. 16B, every response to the polling is the same.
As described above, a UL burst allocated as polling is small in size, but at least occupies radio resources in the UL subframe like a UL burst for normal transmission of UL data. Furthermore, when a UL burst is to be allocated, definition information of the UL burst needs to be added to the UL-MAP by UL-MAP IE. The UL-MAP IE for defining the UL burst is constituted by information including CID (Connection Identifier); UIUC (UL-MAP IE classification); duration (number of slots); and reception coding indication (the number of data repetitions), and has a size of 32 bits (4 bytes). Thus, 10 bytes of radio resources: 4 bytes of UL-MAP IE in the UL-MAP and 6 bytes of UL burst for a BR message, are necessary to perform polling once. In this case, as the polling interval is shorter or as the number of connections is larger, the number of slots used for the polling is larger.
As described above with reference to FIG. 15, both the radio resources for polling and the radio resources for data transmission are in the UL subframe. Therefore, as the amount of radio resources used for polling is larger, the amount of radio resources usable for data transmission is smaller accordingly. In such a case, usage efficiency of UL radio resources for data transmission decreases, which causes a decrease in UL throughput in the entire BS 2.
Such a problem may occur not only in the IEEE 802.16 but also in another radio communication system to perform allocation of radio resources.